The present invention relates to an electrostatic capacity sensor.
An electrostatic capacity sensor which is capable of detecting approach of an object to be detected has been known. A conventional electrostatic capacity sensor is provided with an electrode part and a detection circuit which is connected to the electrode part. The electrode part is constructed from a detection electrode for detecting approach of an object to be detected (hereinafter, this is also referred to simply as xe2x80x9cobjectxe2x80x9d) and a ground electrode grounded to a predetermined part.
In such an electrostatic capacity sensor, the capacitance of the detection electrode is increased as the object approaches the detection electrode. In this way, approach of the object is sensed by detecting the change in the capacitance of the detection electrode with the detection circuit.
Practically In reality, however, in the electrostatic capacity sensor, not only the capacitance of the detection electrode, but also the capacitance of the electrode part and the capacitance of the detection circuit and the like are also detected. Moreover, the capacitance varies with the changes in the ambient conditions such as the temperature and humidity, or vibration, which act as noises in the measurement. Because of this, the conventional electrostatic capacity sensor has low detection precision. Besides, the electrostatic capacity sensor is liable to malfunction when the ambient conditions vary. In particular, when the electrostatic capacity sensor is used in outdoors the detection precision is deteriorated further since the temperature and the humidity vary drastically with the season, time of the day, weather, or the like.
Furthermore, in the conventional electrostatic capacity sensor, the sensitivity of the electrode part itself is low and unstable, so that it has been difficult to increase the detection distance from the detection electrode to an object to be detected (hereinafter, ref erred to as xe2x80x9cdetection distancexe2x80x9d).
In particular, the variations in the capacitance of the electrode part due to the changes in the ambient conditions become larger in the case where the detection plane of the detection electrode has a large area. As a result, noises are increased, so that the detection precision of the electrostatic capacity sensor is reduced. Further, when the area of the detection plane of the detection electrode is relatively large, the threshold of the detection circuit should be set at a high level in order to improve the detection precision. However, this increases influence of noiser so that it is difficult to set the detection distance at a sufficiently large value.
It is an object of the present invention, irrespective of the size of the detection electrode, to provide an electrostatic capacity sensor with high detection precision and high sensitivity.
In order to achieve the above object, the electrostatic capacity sensor according to the present invention comprises an electrostatic capacity type detection element in which a detection electrode, a ground electrode and a charge plate interposed between the detection electrode and the ground electrode are arranged in a state that they are isolated to each other; a detection circuit for detecting changes in capacitance of the detection electrode caused by an object to be detected; and a power supply for supplying electrical power to the detection circuit.
It is preferable that the ground electrode is electrically connected to a negative terminal of the power supply.
The ground electrode may be electrically connected to a negative terminal of the power supply via a relay circuit. In this case, it is preferable that the relay circuit includes a resistor. Further, it is more preferable that the resister is a variable resister.
Moreover, it is preferable that the relay circuit includes a capacitor. Further, it is more preferable that the capacitor is a variable capacitor. In this case, the relay circuit has a function that adjusts sensitivity of the electrostatic capacity sensor. Further, the relay circuit also has a function that stabilizes sensitivity of the electrostatic capacity sensor.
In the electrostatic capacity sensor according to the present invention, the sensitivity of the electrostatic capacity sensor can be adjusted by adjusting the distance between the charge plate and the ground electrode.
It is preferable that the distance between the charge plate and the ground electrode is set so as to be larger than the distance between the detection electrode and the charge plate.
Moreover, it is also preferable that the detection electrode has a first surface that functions as a detection plane and a second surface which is opposite to the first surface, and the detection circuit is provided on the second surface of the detection electrode.
Further, it is also preferable that a plurality of charge plates are arranged along the direction of the thickness of the detection element.